i.MX6ULL驱动开发 | 11 - Linux I2C 驱动框架
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Linux I2C 主机设备驱动分为两部分:
- I2C总线控制器驱动:SOC的I2C控制器外设驱动
- I2C设备驱动:基于I2C总线控制器驱动编写,针对具体的I2C从机设备
一、I2C总线控制器驱动
基于platform平台驱动框架,Linux内核将SOC的I2C外设抽象为i2c_adapter结构体。
1. i2c_adapter结构
i2c_adapter结构体定义在 include/linux/i2c.h
文件中,如下:
/*
* i2c_adapter is the structure used to identify a physical i2c bus along
* with the access algorithms necessary to access it.
*/
struct i2c_adapter
struct module *owner;
unsigned int class; /* classes to allow probing for */
const struct i2c_algorithm *algo; /* the algorithm to access the bus */
void *algo_data;
/* data fields that are valid for all devices */
struct rt_mutex bus_lock;
int timeout; /* in jiffies */
int retries;
struct device dev; /* the adapter device */
int nr;
char name[48];
struct completion dev_released;
struct mutex userspace_clients_lock;
struct list_head userspace_clients;
struct i2c_bus_recovery_info *bus_recovery_info;
const struct i2c_adapter_quirks *quirks;
;
对于一个抽象的i2c_adapter结构体,具体SOC的I2C外设操作函数通过 algo 成员注册。algo成员是 i2c_algorithm 结构体,表示总线访问算法,定义如下:
/**
* struct i2c_algorithm - represent I2C transfer method
* @master_xfer: Issue a set of i2c transactions to the given I2C adapter
* defined by the msgs array, with num messages available to transfer via
* the adapter specified by adap.
* @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
* is not present, then the bus layer will try and convert the SMBus calls
* into I2C transfers instead.
* @functionality: Return the flags that this algorithm/adapter pair supports
* from the I2C_FUNC_* flags.
* @reg_slave: Register given client to I2C slave mode of this adapter
* @unreg_slave: Unregister given client from I2C slave mode of this adapter
*
* The following structs are for those who like to implement new bus drivers:
* i2c_algorithm is the interface to a class of hardware solutions which can
* be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
* to name two of the most common.
*
* The return codes from the @master_xfer field should indicate the type of
* error code that occurred during the transfer, as documented in the kernel
* Documentation file Documentation/i2c/fault-codes.
*/
struct i2c_algorithm
/* If an adapter algorithm can't do I2C-level access, set master_xfer
to NULL. If an adapter algorithm can do SMBus access, set
smbus_xfer. If set to NULL, the SMBus protocol is simulated
using common I2C messages */
/* master_xfer should return the number of messages successfully
processed, or a negative value on error */
int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num);
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, char read_write,
u8 command, int size, union i2c_smbus_data *data);
/* To determine what the adapter supports */
u32 (*functionality) (struct i2c_adapter *);
#if IS_ENABLED(CONFIG_I2C_SLAVE)
int (*reg_slave)(struct i2c_client *client);
int (*unreg_slave)(struct i2c_client *client);
#endif
;
其中重要的成员有两个:
- master_xfer:传输函数,通过此函数完成与IIC从机设备之间的通信
- smbus_xfer:SMBUS总线的传输函数
综上所述,I2C总线驱动的主要工作就是初始化 i2c_adapter 结构体变量,初始化好之后向系统注册可用的i2c_adapter。
一般来说,SOC的I2C总线驱动都是由半导体厂商编写的,对于不同的I2C,通过Linux内核抽象出的 i2c_adapter 即可屏蔽差异,我们只需要基于 i2c_adapter 编写具体的I2C设备驱动即可。
二、I2C设备驱动(重点)
根据总线、设备和驱动模型,I2C设备驱动中,i2c_client 结构体用来描述设备信息,i2c_driver结构体用来描述设备内容。
I2C设备驱动首先要初始化i2c_driver并注册到内核,当设备和驱动匹配以后,i2c_driver中的probe函数会执行,probe函数中需要完成对I2C设备的初始化。
1. i2c_client结构体
一个I2C设备对应一个i2c_client,内核每检测到一个I2C设备就会给该设备分配一个i2c_client。
i2c_client结构体定义在 include/linux/i2c.h
文件中,如下:
/**
* struct i2c_client - represent an I2C slave device
* @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address;
* I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking
* @addr: Address used on the I2C bus connected to the parent adapter.
* @name: Indicates the type of the device, usually a chip name that's
* generic enough to hide second-sourcing and compatible revisions.
* @adapter: manages the bus segment hosting this I2C device
* @dev: Driver model device node for the slave.
* @irq: indicates the IRQ generated by this device (if any)
* @detected: member of an i2c_driver.clients list or i2c-core's
* userspace_devices list
* @slave_cb: Callback when I2C slave mode of an adapter is used. The adapter
* calls it to pass on slave events to the slave driver.
*
* An i2c_client identifies a single device (i.e. chip) connected to an
* i2c bus. The behaviour exposed to Linux is defined by the driver
* managing the device.
*/
struct i2c_client
unsigned short flags; /* div., see below */
unsigned short addr; /* chip address - NOTE: 7bit */
/* addresses are stored in the */
/* _LOWER_ 7 bits */
char name[I2C_NAME_SIZE];
struct i2c_adapter *adapter; /* the adapter we sit on */
struct device dev; /* the device structure */
int irq; /* irq issued by device */
struct list_head detected;
#if IS_ENABLED(CONFIG_I2C_SLAVE)
i2c_slave_cb_t slave_cb; /* callback for slave mode */
#endif
;
- flag:标志
- addr:I2C从机地址,7bit
- name:i2c设备名称
- adapter:对应的I2C总线控制器
- dev:设备结构体
- irq:中断
- detected
2. i2c_driver结构体
i2c_driver结构体定义在include/linux/i2c.h
文件中,如下:
/**
* struct i2c_driver - represent an I2C device driver
* @class: What kind of i2c device we instantiate (for detect)
* @attach_adapter: Callback for bus addition (deprecated)
* @probe: Callback for device binding
* @remove: Callback for device unbinding
* @shutdown: Callback for device shutdown
* @alert: Alert callback, for example for the SMBus alert protocol
* @command: Callback for bus-wide signaling (optional)
* @driver: Device driver model driver
* @id_table: List of I2C devices supported by this driver
* @detect: Callback for device detection
* @address_list: The I2C addresses to probe (for detect)
* @clients: List of detected clients we created (for i2c-core use only)
*
* The driver.owner field should be set to the module owner of this driver.
* The driver.name field should be set to the name of this driver.
*
* For automatic device detection, both @detect and @address_list must
* be defined. @class should also be set, otherwise only devices forced
* with module parameters will be created. The detect function must
* fill at least the name field of the i2c_board_info structure it is
* handed upon successful detection, and possibly also the flags field.
*
* If @detect is missing, the driver will still work fine for enumerated
* devices. Detected devices simply won't be supported. This is expected
* for the many I2C/SMBus devices which can't be detected reliably, and
* the ones which can always be enumerated in practice.
*
* The i2c_client structure which is handed to the @detect callback is
* not a real i2c_client. It is initialized just enough so that you can
* call i2c_smbus_read_byte_data and friends on it. Don't do anything
* else with it. In particular, calling dev_dbg and friends on it is
* not allowed.
*/
struct i2c_driver
unsigned int class;
/* Notifies the driver that a new bus has appeared. You should avoid
* using this, it will be removed in a near future.
*/
int (*attach_adapter)(struct i2c_adapter *) __deprecated;
/* Standard driver model interfaces */
int (*probe)(struct i2c_client *, const struct i2c_device_id *);
int (*remove)(struct i2c_client *);
/* driver model interfaces that don't relate to enumeration */
void (*shutdown)(struct i2c_client *);
/* Alert callback, for example for the SMBus alert protocol.
* The format and meaning of the data value depends on the protocol.
* For the SMBus alert protocol, there is a single bit of data passed
* as the alert response's low bit ("event flag").
*/
void (*alert)(struct i2c_client *, unsigned int data);
/* a ioctl like command that can be used to perform specific functions
* with the device.
*/
int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);
struct device_driver driver;
const struct i2c_device_id *id_table;
/* Device detection callback for automatic device creation */
int (*detect)(struct i2c_client *, struct i2c_board_info *);
const unsigned short *address_list;
struct list_head clients;
;
3. 注册/删除API
extern int i2c_register_driver(struct module *, struct i2c_driver *);
extern void i2c_del_driver(struct i2c_driver *);
由宏定义 CONFIG_I2C
和 CONFIG_I2C_MODULE
控制。
注册也可以用再封装的宏:
/* use a define to avoid include chaining to get THIS_MODULE */
#define i2c_add_driver(driver) \\
i2c_register_driver(THIS_MODULE, driver)
三、设备树如何描述I2C设备
1. SOC的i2c外设
这部分描述是芯片原厂写的,在arch/arm/boot/dts/imx6ull.dtsi
文件中:
2. I2C设备描述
这部分描述在开发板描述文件中,用来描述i2c总线上外接了哪些设备,在arch/arm/boot/dts/imx6ull-atk-emmc.dts
文件中:
可以看到 NXP 官方的EVK开发板在I2C1上接了两个设备,mag3110是一个磁力计,fxls是一个加速度计。
- 节点名称:这里@后的地址表示I2C设备地址
- compatible:兼容性
- reg:设置I2C设备地址
- position
四、I2C设备数据收发
1. I2C数据传输函数
i2c_transfer函数的原型如下:
/* Transfer num messages.
*/
extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num);
函数参数意义如下:
- adap:I2C设备所使用的总线控制器,i2c_client结构体中保存了其对应的i2c_adapter;
- msgs:I2C设备要发送的一个或多个消息;
- num:msgs的消息数量
函数返回值表示成功发送的msgs数量,否则为负值。
该函数最终会调用的 adap 中对应的 master_xfer 函数。
2. I2C数据传输消息
i2c_msg是一个结构体类型,定义在include/uapi/linux/i2c.h
文件中,如下:
/**
* struct i2c_msg - an I2C transaction segment beginning with START
* @addr: Slave address, either seven or ten bits. When this is a ten
* bit address, I2C_M_TEN must be set in @flags and the adapter
* must support I2C_FUNC_10BIT_ADDR.
* @flags: I2C_M_RD is handled by all adapters. No other flags may be
* provided unless the adapter exported the relevant I2C_FUNC_*
* flags through i2c_check_functionality().
* @len: Number of data bytes in @buf being read from or written to the
* I2C slave address. For read transactions where I2C_M_RECV_LEN
* is set, the caller guarantees that this buffer can hold up to
* 32 bytes in addition to the initial length byte sent by the
* slave (plus, if used, the SMBus PEC); and this value will be
* incremented by the number of block data bytes received.
* @buf: The buffer into which data is read, or from which it's written.
*
* An i2c_msg is the low level representation of one segment of an I2C
* transaction. It is visible to drivers in the @i2c_transfer() procedure,
* to userspace from i2c-dev, and to I2C adapter drivers through the
* @i2c_adapter.@master_xfer() method.
*
* Except when I2C "protocol mangling" is used, all I2C adapters implement
* the standard rules for I2C transactions. Each transaction begins with a
* START. That is followed by the slave address, and a bit encoding read
* versus write. Then follow all the data bytes, possibly including a byte
* with SMBus PEC. The transfer terminates with a NAK, or when all those
* bytes have been transferred and ACKed. If this is the last message in a
* group, it is followed by a STOP. Otherwise it is followed by the next
* @i2c_msg transaction segment, beginning with a (repeated) START.
*
* Alternatively, when the adapter supports I2C_FUNC_PROTOCOL_MANGLING then
* passing certain @flags may have changed those standard protocol behaviors.
* Those flags are only for use with broken/nonconforming slaves, and with
* adapters which are known to support the specific mangling options they
* need (one or more of IGNORE_NAK, NO_RD_ACK, NOSTART, and REV_DIR_ADDR).
*/
struct i2c_msg
__u16 addr; /* slave address */
__u16 flags;
#define I2C_M_TEN 0x0010 /* this is a ten bit chip address */
#define I2C_M_RD 0x0001 /* read data, from slave to master */
#define I2C_M_STOP 0x8000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_NOSTART */
#define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */
__u16 len; /* msg length */
__u8 *buf; /* pointer to msg data */
;
- addr:从机地址
- flags:标志
- len:该msg消息长度
- buf:消息内容
3. 进一步封装的API
/*
* The master routines are the ones normally used to transmit data to devices
* on a bus (or read from them). Apart from two basic transfer functions to
* transmit one message at a time, a more complex version can be used to
* transmit an arbitrary number of messages without interruption.
* @count must be be less than 64k since msg.len is u16.
*/
extern int i2c_master_send(const struct i2c_client *client, const char *buf,
int count);
extern int i2c_master_recv(const struct i2c_client *client, char *buf,
int count);
函数参数意义如下:
- client:I2C设备对应的i2c_client
- buf:要发送的数据
- count:要发送的数据字节数,要小于64KB(i2c_msg的len成员是一个u16类型)
函数返回值表示成功发送的字节数,否则返回负值。
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